Windmills have been used for many generations for the purpose of pumping water from the ground and for generating electricity. The basic advantage of the windmill is that it uses the power of the wind to rotate a wheel having radially extending blades that are driven by the wind. This rotary movement may be converted into various useful purposes. For example, wind turbines in the form of propellers mounted on towers have been placed in areas where steady winds are prevalent and the wind turbines are used to generate electricity.
The blades of the conventional wind turbines are very large and made of expensive rigid material and are constructed to have the blades extend radially from a central hub, with no extra support at the outer tips of the blades. The conventional wind turbine blades rotate at a high rate of revolution and must withstand both the centrifugal forces generated by the fast revolution of the blades and the cantilever bending forces applied to the blades by the wind. Since the outer portions of the blades may be engaged by strong winds moving at a very high velocity, the larger the blades the stronger they must be and the more expensive they become. Thus, there is a practical limit as to the length and width of the blades because of the expense of stronger materials for larger blades.
The prior art shows some turbine wheels that have been constructed with circular rims that support the blades at the outer ends of the blades. This tends to reduce the stress applied by the wind to the blades, but the circular rims add more weight to the structure and present more wind resistance that increases the tipping forces applied to the mast that supports the turbine wheel.
Another type of wind turbine is one that has blades in the form of sail wings constructed of cloth that are a substitute for the rigid blades of the conventional wind turbines described above. For example U.S. Pat. Nos. 4,330,714, 4,350,895, and 4,729,716 disclose wind turbines that do not use rigid propeller blades but use sails that catch the wind. The sails are mounted on radiating spars of the turbine. These particular wind turbines include circular inner and outer rims with the sails of the turbine supported by both the inner and outer rims. The outer rim supports the outer portions of the sails so that the axial force of the wind applied to the sails may be absorbed to a major extent by the outer rim so there is little if any cantilever force applied to the sails. This allows the blades of the wind turbine to be formed of lighter weight material, material that is not required to bear as much stress in comparison to the typical free bladed turbine.
It appears that the above described prior art wind turbines do not have adequate control of the pitch of the blades for adjusting the pitch when the wind velocity changes or when starting or stopping the rotation of the turbine wheel.
The adjustment of the pitch of the turbine blades is important in order to adjust to the different atmospheric wind speeds. An important factor in a well designed airfoil's efficiency is its angle of attack to the apparent wind. The apparent wind is the direction of the wind that is blowing across the turbine blade as the blade rotates.
A blade is designed with a twist or “pitch” so that the leading edge will be at the ideal angle of attack to the apparent wind along the entire leading edge of the blade at a given wind speed. The apparent wind near the tip of the blade is faster than at the root of the blade and is at a different angle than the wind at the root of the blade. Whenever the wind is blowing faster or slower than the designed blade wind speed, the angle of the blade may be controlled so as, to adjust the blade to get the best angle of attack near the tip of the blade. The tip of the blade is where the greater amount of power is harvested, and where the most sweep area or width of the blade is present. However, this structure may put the lower part of the blade at a wrong angle and therefore reduce the lift of this portion of the blade. If the angle of attack of the blade with respect to the apparent wind gets too far off, the blade might begin to stall from the root up to the tip, creating drag at the root rather than lift. Therefore it is desirable to construct the blades of a wind turbine so they can change pitch for different wind conditions.
Also, including an outer rim about extra turbine blades adds weight to the perimeter of the turbine wheel.
Thus, it would be desirable to provide a wind turbine that has support for the outer ends of the turbine blades. It also would be desirable to provide a wind turbine that has improved variable pitch blades for controlling the speed of rotation of the turbine wheel, and desirable to provide blade extensions for increasing the ability of the wind turbine to catch more wind, and to be able to vary the pitch of the blade extensions.